1. Field of the Invention:
The present invention relates to the preparation of photopolymerizable, liquid, storage stable organopolysiloxane compositions and, more especially, to the preparation of such organopolysiloxanes by reaction between an .alpha.,.omega.-dihydroxydiorganopolysiloxane oil and an alkoxysilane functional monomer, said alkoxysilane bearing functional groups directly covalently bonded to the silicon atoms thereof, and such functional groups being selected from the group consisting of methacryloxyalkyl, acryloxyalkyl and mercaptoalkyl radicals.
2. Description of the Prior Art:
It is known to this art to prepare liquid organopolysiloxanes by reaction between an hydroxylated organopolysiloxane and an alkoxysilane functional monomer bearing, directly covalently bonded to the silicon atoms thereof, methacryloxyalkyl or acryloxyalkyl functional groups. Compare French Pat. Nos. 1,522,607 and 2,110,115; U.S. Pat. No. 3,782,940. Nonetheless, the operating conditions set forth in the foregoing prior art patents for their respective processes are insufficiently exacting as to enable repeatable and/or reliable production of such organopolysiloxanes, especially of those which are both storage stable and can be used in the absence of solvents therefor; not atypically polymers are prepared which gel upon storage and/or which (after the addition of photosensitizers thereto) are cured into coatings exhibiting but average, and ofttimes inferior, anti-stick or release properties. And this in spite of the fact that exceedingly long periods of exposure to curing radiation be employed, e.g., exposure times in excess of 50 seconds.
The published Japanese application, Kokai No. 78/118,500, also described a process for the reaction of .alpha.,.omega.-dihydroxypolysiloxanes with trialkoxysilanes bearing methacryloxyalkyl or acryloxyalkyl functional groups, at elevated temperatures and in the presence of an alcohol having a boiling point at least 150.degree. C. higher than that of the aliphatic alcohol generated during the reaction. However, the process has the disadvantages of being quite slow and yielding condensates having a low viscosity and which only very slowly cross-link under the influence UV radiation.
Organopolysiloxanes related in structure to those noted hereinabove, but prepared by yet another process, are described in published French application No. 2,192,137. Such process includes reacting, in an aprotic solvent and in the presence of a basic catalyst, an alkoxysilane bearing methacryloxyalkyl or acryloxyalkyl functional groups, with a cyclic or linear organopolysiloxane. This method requires the use of solvents which is always a marked disadvantage on an industrial scale; it further entails the rearrangement of the siloxane bonds, which leads to a very complex mixture of polymers.
Certain photopolymerizable compositions which find use in the treatment of paper to impart anti-stick properties thereto, and which comprise, in addition to photosensitizers, diorganopolysiloxane moieties including unsaturated ethylene radicals, and particularly methacryloxypropyl radicals, too have been disclosed to the art, in U.S. Pat. No. 3,726,710. These methacryloxypropyl radicals are principally attached along the chain of the diorganopolysiloxane polymers and not at the ends of the chain. To introduce such radicals along the chain, or polymer backbone, it is necessary to employ a method that is considerably more complex than that consisting of simply reacting .alpha.,.omega.-dihydroxydiorganopolysiloxanes with alkoxysilanes bearing methacryloxypropyl radicals. The text of the patent does not provide detailed information concerning specific methods that may be used; nevertheless, any process effecting conventional substitution would be appropriate.
Such process, thus, may consist of preparing, in a first stage, .alpha.,.omega.-bis(triorganosiloxy)-diorganopolysiloxane polymers (not readily available on the silicone market) bearing pendant chloroalkyl radicals along the polymer chains bonded to silicon atoms and subsequently reacting, in a solvent and in the presence of an HCl acceptor, the chloroalkyl radicals with methacrylic and acrylic acids. Such a process is not only time consuming, but requires an inordinate degree of care to obtain a high rate of substitution.
It is also known to prepare other types of liquid organopolysiloxanes by the reaction of .alpha.,.omega.-dihydroxydiorganopolysiloxanes with alkoxysilanes bearing mercaptoalkyl functional groups; U.S. Pat. No. 4,066,603. In this process, the reactants are maintained in contact, in the presence of a basic catalyst such as caustic potash or a potassium silanolate, and the alcohol formed during the reaction is completely eliminated via a devolatilization stage; the use of this type of catalyst suggests, at the completion of the reaction, the employment of a neutralization stage (in order to prevent product evolution during storage) and likely a filtration stage. Such a process is indeed complex and in need of improvement.
Organopolysiloxanes which are photopolymerizable upon the addition of photosensitizers thereto, and comprised of organopolysiloxanes bearing alkenyl radicals bonded to silicon atoms, even organopolysiloxane polymers comprising SiH moieties and silanes or organopolysilanes bearing mercaptoalkyl functional groups, are described in numerous patents, e.g., French Pat. Nos. 2,245,729, 2,246,608, 2,257,736 and 2,622,126. Such compositions are capable of being processed into adequate anti-stick coatings, but same do not as yet possess all of the necessary properties as would favor their industrial development. Such properties would have to comprise at least the following: ease of preparation of the various components; good, mutual compatibility of the several components; low viscosity; and rapid hardening or curing of the polymers upon exposure to ultraviolet radiation.